a) Field of the Invention
The present invention relates to a technique for developing resist at high precision.
b) Description of the Related Art
Forming a resist pattern at high precision has long been desired as semiconductor integrated circuits have become highly integrated. In this specification, a term "resist" collectively means any resist sensitive to energy beams, such as photoresist and electron beam (EB) resist.
Positive type resist and negative type resist Bare known. Of the positive type resist, the region where an energy beam such as a light beam and electron beam was applied is removed. Of the negative type resist, the region where an energy beam was not applied is removed.
EB resist of high resolution will be explained illustratively. Generally used as EB resist are styrene based or methacrylic acid based positive resist, and styrene based negative resist. EB resist is generally coated by a spinner on the surface of an underlie such as a reticle quartz substrate and a Si substrate, and exposed by an electron beam.
An EB resist film after being exposed undergoes a development process. This development process generally includes:
(1) a development process, PA1 (2) a rinse process, and PA1 (3) a dry process.
In the development process, an EB resist film is selectively dissolved by organic solvent by a chemical property difference between the exposed region and unexposed region of the EB resist film. Developing solution not only selectively dissolves the exposed region or unexposed region, but also swells the unremoved EB resist film.
In the rinse process, the developing solution is supplanted by rinsing solution. The rinse process has a function of tightening and condensing the resist film swelled by the developing solution as well as the function of stopping the development. As the rinsing solution for EB resist, organic solvent having a lower resist solubility than developing solution is generally used.
For example, for positive type EB resist, there are used a mixed solution of methylethylketone and methylisobutylketone as the developing solution and a mixed solution of isopropylalcohol and methylisobutylketone as the rinsing solution. For negative type EB resist, there are used a mixed solution of isoamylacetate and ethylene glycol monoethyl ether as the developing solution and ethylene glycol monoethyl ether as the rinsing solution.
After the development is stopped and the original condensed state of the swelled resist film is recovered, the resist film is dried to complete the developing program.
A new defect has been found recently during a resist development process. Namely, precipitate as thick as 1 .mu.m is left in a hole formed by the development process. This phenomenon is conspicuous particularly in the case of positive type resist. The cause of generating such a defect may be reasoned From the following.
In the developing process, resist is dissolved into developing solution. Developing solution has a high solubility relative to resist. Positive type EB resist after the development process reduces its thickness, and the ratio of a finished film thickness to the original film thickness is only about 90%. The solubility of negative type EB resist into developing solution is lower than that of positive type EB resist. However, the ratio is in the order not negligible.
During the transition period from the development process to the rinse process, the developing solution and rinsing solution contact each other and are mixed together. Part of the resist components dissolved greatly in the developing solution is immersed in the rinsing solution and is deposited as precipitate again on the surface of the underlie, because of a low solubility of the rinsing solution.
For example, if precipitate having a diameter of about 1 .mu.m is deposited in a window formed in a reticle, a defect of about 0.2 .mu.m is formed on a subject exposed at a reduction ratio of 1/5. This defect of about 0.2 .mu.m is fatal to a highly integrated semiconductor device.
Such a defect is formed not only on negative type resist but also on positive type resist, and not only on EB resist but also on photoresist.
A defect caused by resist re-deposition is formed due to a solubility difference between developing solution and rinsing solution. In order to relieve an abrupt change in the solubility, it is conceivable that a plurality of rinsing solutions or a plurality of mixed solutions of developing and rinsing solutions at different compositions are prepared to transit form the developing process to the rinsing process while gradually changing the resist solubility.
As described above, a surface defect by resist redeposition on the reticle surface is caused by a solubility difference between developing and rinsing solutions.